Reinforced synthetic enamel coating for electrical conductor



April 3, 1962 w. w. WAREHAM 3,028,265

REINFORCED SYNTHETIC ENAMEL COATING FOR ELECTRICAL CONDUCTOR Filed May 6, 1957 [/2 1 677 tor:

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His ,4 ziztorney United States PatentO 3,028,265 REINFORCED SYNTHETIC ENAMEL COATING FOR ELECTRICAL CONDUCTOR William W. Wareham, Fort Wayne, Iud., assignor to General Electric Company, a corporation of New York Filed May 6, 1957, Ser. No. 657,455 3 Claims. (Cl. 117232) This application is a continuation-in-part of copending application Serial Number 465,552, filed October 29, 1954, now abandoned.

This invention relates to synthetic enamels, and more particularly to reinforced synthetic enamels of the type having insulating qualities and utilized as insulation for electrical conductors such as magnet wire.

It is a well-known fact that electrical conductors such as magnet wire require a coating of insulation to prevent short circuits since such wire is almost always wound upon itself, and is often positioned in contact with various other conductive parts of electrical apparatus. The high quantity of wire used in many applications requires that the coating be a relatively thin one and that it be relatively simple to apply. In this connection, many synthetic enamels have been discovered which suit the purpose admirably. However, it frequently occurs that the enamels do not have the physical properties required and that, therefore, they will not provide sufficient resistance to abrasion once they are coated on the wire. This may, of course, cause a bare spot in the coating and thereby increase the chances of short circuits and faulty operation of the apparatus in which the wire is used. Another feature of many synthetic enamels is that they have a strong tendency toward plastic flow. It is most desirable to eliminate the feature of plastic flow as much as possible since such an occurrence will damage the insulating properties by causing an uneven coating of insulation and to provide a positive spacing factor between conductors even when the resin coating is injured. The great amount of bending to which magnet wire is subjected makes it necessary to achieve the above advances without any substantial sacrifice of the flexibility of the synthetic enamel coating.

It is, therefore, an object of this invention to provide a synthetic enamel, for use as a coating for electrical conductors, which will incorporate the desirable features set forth above.

Further objects and advantages of this invention will become apparent and the invention will be better understood by reference to the following description and the accompanying drawing, and the features of novelty which characterize this invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

This invention, in its broadest aspect, provides a synthetic insulating enamel of the type adapted to be used to cover electrical conductors such as magnet wire. Combined with this is a predetermined amount of glass in fibrous form dispersed through the enamel. The amount of glass is sufiicient so that the fibers provide an interlacing eifect thereby to resist plastic flow and abrasion of the enamel. In addition, the glass acts as an inorganic spacer which keeps the individual conductors from touching each other under environmental and service conditions which might destroy the organic resin of the insulation.

In the drawing, the single figure is a cross sectional view of a conductor provided with a coating of the improved enamel.

Referring now to the drawing, there is shown an electrical conductor 1 covered with an insulating coating 2 which is provided in accordance with the teaching of this ice invention. It will of course, be understood that the drawing is illustrative and is not intended to suggest particular thicknesses of the conductor, and the coating 2. Coating 2 is an insulating synthetic enamel reinforced with a fibrous glass filler, as indicated by the stippling in the drawing. In order to provide a clearer understanding of the invention, the drawing will be explained using a particular type of enamel for illustrative purposes. A most appropriate type of insulation, in view of its wellknown lack of resistance to abrasion and its tendency to plastic flow, is polytetrafluoroethylene. This material generally comes in a slurry made up of about 50 to 60 percent by weight of polytetrafluoroethylene suspended in water. For this particular type of synthetic enamel, the fibrous glass would be mixed with the enamel directly, that is, the two would simply be combined together;

however, it will be understood that for many types of enamel used, with the maximum varying for dilferent enamels. The best results are achieved, for polytetra fluoroethylene when approximately 15 percent fibrous glass is present; with this proportion there is high resistance to abrasion and plastic flow, and the flexibility of the enamel coating is substantially unchanged. The effect of the glass fibers is achieved through the fact that the fibers provide an interlacing action which both reinforces the material of the enamel itself and impedes plastic flow.

The optimum diameter of glass fibers has been found to be in the range of 0.05 to 1.0 micron; the length of the fibers ranges from approximately 0.2 micron to several microns with most of the fibers in the 5 to 25 micron range. Whilethe diameters of the fibers may be constant or varied, it is desirable that the length of the fibers actually vary between the two limits set forth in order to obtain the maximum interlacing and reinforcing effect. Fibers having diameters of 0.05 micron and up are commercially available, and it is a simple matter to achieve the desired length of these fibers either by crushing or by ball milling, as is well known in the art.

It will of course be understood that the figures given herein are merely illustrative of the optimum dimensions and that other groupings of dimensions may be used with varying success. For instance, it is possible to exceed the 25 percent upper limit of the amount of glass for polytetrafluoroethylene by decreasing the diameter of the fibers, since the smaller the diameter, the greater the amount of glass fiber filler which may be added without decreasing the flexibility to any appreciable extent.

Once an enamel has been mxed with glass fiber filler as desired, it is then a simple matter to apply the enamel as a coating to a conductor in a manner well known in the art. To illustrate the applications of the invention, reference is made to the following examples:

Example I A standard slurry of polytetrafiuoroethylene suspended in water was mixed with fibrous glass having an average diameter of 0.5 micron and a length of about 0.5 micron to several microns; the mixture, by weight, consisted of 15 percent glass fibers and percent polytetrafluoroethylene solids in a 50 percent solids slurry. A conductor was then passed through the mixture six times to provide six coats, with approximately one minute of baking being provided between each coat. Each baking started at about 115 degrees centigrade and increased to about 465 degrees centigrade before the wire passed out of the heating oven. Examination of the material showed that there was a marked improvement in the resistance of the polytetrafluoroethylene coating to abrasion and plastic flow without any harmful decrease in the flexibility of the coating.

Example 2 A phenolic-modified polyvinyl formal resin was mixed With fibrous glass having an average diameter of 0.5 micron. The glass was first crushed under high pressure (9000 pounds per square inch or more) then mixed with cresylic acid, and then ball milled so that the lengths of the fibers were decreased to a range of 0.5 micron to several microns. The glass and resin were then mixed, the mixture consisting by weight of 20 percent glass fibers and 80 percent resin. The mixture was then applied to a conductor in 6 passes with a 15 second bake at 325 degrees centigrade provided after each pass. Again, a marked improvement in the resistance to abrasion was observed without any substantial eifect on the flexibility of the wire. The burnout time for a three layer coil of .0159 inch diameter magnet wire exposed to a current density of 65,000 amps/in. was increased from 89.6 seconds to 184 seconds by adding glass fibers.

Example 3 An aqueous dispersion of bntyl acrylate and acrylonitrile was mixed with fibrous glass by first mixing 25 percent by weight of glass with water then adding it to the butyl acrylate and acrylonitrile so that the resin finally contained 20 percent by weight of glass of the same dimensions as in the previous examples. In this case, no substantial effect on the resistance to abrasion of the resin was observed. However, as before, there was a noticeable improvement in the burnout time which Went from an average of 58 seconds to an average of 73 seconds for the same test as set forth above.

It will be seen from the foregoing that this invention provides a synthetic enamel coating for electrical conductors which is reinforced so that it has greatly increased resistance to abrasion and to. plastic fiow, or improved performance characteristics under severe heat and pressure conditions, or both, with practically no detrimental effect insofar as the flexibility is concerned.

While this invention has been explained by describing a particular embodiment thereof, it Will be apparent that improvements and modifications may be made without departing from the scope of the invention as defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical conductor provided with an insulating coating therefor comprising polytetrafluoroethylene, and glass fibers dispersed through said polytetrafluoroethylene and forming approximately five to twenty-five percent by weight of the same, said glass fibers having diameters in the range of approximately 0.05 to 0.5 micron and having lengths varying from approximately 0.2 micron to several microns, said length of said fibers being substantially greater than said diameters.

2. An electrical conductor provided with an insulating coating therefor comprising phenolic-modified polyvinyl formal resin, and glass fibers dispersed through said resin and forming approximately twenty percent by weight of the same, said glass fibers having diameters in the range of approximately 0.05 to 0.5 micron and having lengths varying from approximately 0.2 micron to several 1 microns, said length of said fibers being substantially greater than the diameter thereof.

3. An electrical conductor provided with an insulating coating therefor comprising: a member selected from the group consisting of polytetrafluoroethylene, phenolicmodified polyvinyl formal resin, and a combination of bntyl acrylate and acrylonitride; and glass fibers dispersed through said member and forming approximately five to twenty-five percent by Weight of the member, said glass fibers having diameters in the range of approximately 0.05 to 0.5 micron and having lengths varying from approximately 0.2 micron to several microns, said length of said fibers being substantially greater than the diameter theerof.

References Cited in the file of this patent UNITED STATES PATENTS 2,333,513 Berberich et al. Nov. 2, 1943 2,400,099 Brubaker et al. May 14, 1946 2,459,653 Keyes Jan. 18, 1949 2,478,322 Robinson et al. Aug. 9, 1949 2,498,785 Bennett et al Feb. 28, 1950 2,549,017 Saffir Apr. 17, 1951 2,591,383 Spalding Apr. 1, 1952 2,691,694 Young Oct. 12, 1954 2,707,703 Dorst May 3, 1955 FOREIGN PATENTS 1,031,164 France June 22, 1953 618,094 Great Britain Feb. 16, 1949 

1. AN ELECTRICAL CONDUCTOR PROVIDED WITH AN NSULTING COATING THEREFOR COMPRISING POLYTETRAFLUOROETHYLENE, AND GLASS FIBERS DISPERSED THROUGH SAID POLYTETRAFLUOROETHYLENE AND FORMING APPROXIMATELY FIVE TO TWENTY-FIVE PERCENT BY WEIGHT OF THE SAME, SAID GLASS FIBERS HAVING DIAMETERS IN THE RANGE OF APPROXIMATELY 0.05 TO 0.5 MICRON AND HAVING LENGHTS VARING FROM APPROXIMATELY 0.2 MICRON TO SEVERAL MICRONS, SAID LENGHT OF SAID FIBERS BEING SUBSTANTIALLY GREATER THAN SAID DIAMETERS. 